You know that what is "Physical Aspects" ?

In mainstream first generation IBM personal computers (that is the PC,XT and AT) motherboards fit one of three different  sizes each defined by the machine's initials.in the past,the motherboards of all PC's measured about 8 1/2 inches.the XT  strectched nearly an inch longer the AT in need of more space for more circuity extended all the way to 12 by 13 1/2 inches.lower priced commodity computers generally followed the same scheme allowing for board manufacturers and ase maker to offer compatible products,you can be sure that a clone of an XT motherboard will fit an XT clone case.IBM set the pattern compatible makers merely followed it.
Major compatible manufacturers -compaq Espon,leading edge,Tandy and so on control the manufacture of both system board and case they have the freedom to vary dimensions as may be required for example the original compaq portable motherboard was some what larger than that of the PC.